All electronic devices, especially also field devices of automation technology, require a power supply.
Field devices of automation technology are applied for registering and/or influencing process variables. Examples of such field devices include fill level measuring devices, mass flow measuring devices, pressure- and temperature measuring devices, pH-redox-potential-measuring devices, conductivity measuring devices, etc., which, as sensors, register the corresponding process variables, fill level, flow, pressure, temperature, pH-value and conductivity value, respectively. Serving for influencing process variables are so-called actuators, e.g. valves, which control the flow of a liquid in a pipeline section or pumps, which change the fill level in a container. Referred to as field devices are, thus, in principle, all devices, which are applied near to the process and deliver, or process, process relevant information. In connection with the invention, the terminology, field device, thus, includes all types of measuring devices and actuators, as well as e.g. gateways, radio adapters and other bus participants integrated/integrable in a bus system.
A large number of such field devices are manufactured and sold by the group of firms, Endress+Hauser.
As already mentioned, such field devices require a power supply. Known from the state of the art are essentially three variants for supplying devices, respectively field devices, with electrical power from the grid. In the case of all three variants, in the case of grid operation, a galvanic isolation by means of at least one transformer is necessary. The simplest case of a rectification is that, in which a grid transformer (50 Hz) is applied with a following linear voltage regulator. The efficiency of such a circuit is, however, relatively poor. In order to reduce these losses, the linear voltage regulator is replaced by a switching regulator. In this way, an efficiency from 50-70% can be achieved. Because the switching regulator is located on the secondary side of the grid transformer, such circuits are also referred to as secondary switched power supplies. The efficiency of these secondary switched power supplies can be further increased through use of a high-frequency transformer (typically some kHz up to a MHz). However, the technical effort in the case of this circuit form is the greatest. First, the grid voltage must be rectified, then converted into a high-frequency alternating voltage, in order to be able to then transform it. For a desired output voltage, it is additionally still necessary to control the pulse width. Used for this is a control loop, which is embodied between the secondary side and the primary side. In order further to have a galvanic isolation, a potential isolation, for example, an optocoupler, is built into the control loop. Such circuit forms are also referred to as primary switched power supplies. These primary switched power supplies have typically an input voltage range, which extends over a factor of about 5, for example, from 20 to 100V, wherein in the low input voltage region, typically about 20 to 40V, the efficiency is, however, as low, as 60%.